There has been known in the art such a type of keyboard musical instrument which comprises light emitting elements respectively provided in correspondence to the individual music-playing keys to make illuminative displays in association with the individual key actuations, as disclosed, for example, in unexamined Japanese patent publication No. 2003-99067 in which visual indicators emit light in varying color or intensity according to the respective tone properties of the corresponding keys, and in unexamined Japanese patent publication No. H10-222160 in which light emitting diodes flash according to the key actuation by the player as a performance display or according to the music teaching data as a guidance to the student. The former publication discloses a keyboard musical instrument having indicator elements which are arrayed horizontally on the back of the instrument individually corresponding to the individual keys and emit light in intensity representing the velocity or strength of the depressed keys.
With the keyboard musical instrument disclosed in the former publication, however, the depression strength of each key is expressed by the light intensity of each corresponding light emitting element among those arrayed in a line horizontally, and it is accordingly rather difficult for the user (or player) to know precise intensities of each key depression strength. In addition, the user cannot compare his/her key depression strengths with those of the exemplary performance.